Lighting of incandescent lamps or light bulbs in, for example, automotive applications typically involves driving a twelve volt (12V) filament. In the past, 12V low beam incandescent lights were switched on via a relay which connected the battery voltage directly across the lights, wherein the current through the filament depended on its resistance. Typically, the resistance of a cold filament can be ten times less than the resistance of a hot filament. As a result, the inrush current can be ten times the running current of a light, leading to thermal stress on the light filament and premature failure. With low-cost incandescent lamps, this shortened lifetime has not been a problem. However, with the advent of higher cost lights, such as halogen lamps for example, it is desirable to lengthen the life of the lamp where possible.
One solution for extending lamp life is to limit the inrush current to the lamp by adding current limiting circuitry. The current limiting circuitry, if linear, adds dissipation and can fail if the lamp is shorted. The linear circuitry also lowers the voltage to the lamp, which in a halogen lamp reduces life if the filament is not hot enough to initiate the “Halogen Cycle”. Sustained low voltage on a halogen lamp can cause blacking of the glass and filament breakage due to filament diameter reduction. Another method of current limiting is a high side switch which pulse-width-modulates the battery voltage to the lamp during the inrush period. After the inrush period is over and the filament is hot, the high side switch is turned on continuously, wherein dissipation is lower but there is no regulation of the lamp when the battery voltage dips again resulting in reduced lamp life. Other lighting solutions provide for different lamp configuration such as metal halide/Xenon and high intensity discharge (HID) discharge lamps. However, these other lamp systems have high voltage requirements, which can exhibit electromagnetic interference, and complex control circuitry, which adds significant cost to the lighting system. In addition, pulse width modulation voltage control can cause electromagnetic interference (EMI) with radio equipment in the vehicle as well as interference with other communication devices near the vehicle.
Therefore, there is a need in the art for a cost effective and efficient way to control current in a halogen lamp.